Giant magnetoresistive (GMR) or tunneling magnetoresistive (TMR) films typically include at least a tri-layered film of: a fixed magnetic layer, a non-magnetic layer next to the fixed magnetic layer, and a free magnetic layer on the opposite side of the non-magnetic layer. A GMR or TMR film has a low resistance when the polarization of the free magnetic layer is parallel to the polarization of the fixed magnetic layer, and the GMR or TMR film has a high resistance when the polarization of the free magnetic layer is anti-parallel to the polarization of the fixed magnetic layer. The resistance of the GMR or TMR film can oscillate from its high resistance to its low resistance and vice versa when the polarization of the free magnetic layer oscillates relative to the polarization of the fixed magnetic layer.
Within the past few years, oscillators have been created using the oscillating resistor characteristic of GMR and TMR films. These oscillators pass a current through a single submicrometer contact to the GMR or TMR film, which can be a conventional film or a patterned nanopillar structure. The current becomes spin-polarized while passing through the tri-layered GMR or TMR film, and then, by conserving angular momentum, the spin-polarized current exerts a torque on the GMR or TMR film. In an applied magnetic field, the spin-transfer torque can produce a steady-state precession of the magnetization of the free magnetic layer at microwave frequencies. As a result, the oscillator produces spin-transfer-induced oscillations.
These single submicrometer contact, spin-transfer oscillators have a significant disadvantage because they can generate only very limited amounts of output power, typically in the lower nanoWatt (nW) range. To be useful in practical device applications, however, these oscillators need to output as much power as possible, preferably in the microWatt (μW) range. Accordingly, a need exists for a technique to increase the output power for spin-transfer oscillators.
For simplicity and clarity of illustration, the drawing figures illustrate the general manner of construction, and descriptions and details of well-known features and techniques may be omitted to avoid unnecessarily obscuring the invention. Additionally, elements in the drawing figures are not necessarily drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve understanding of embodiments of the present invention. The same reference numerals in different figures denote the same elements.
The terms “first,” “second,” “third,” “fourth,” and the like in the description and in the claims, if any, are used for distinguishing between similar elements and not necessarily for describing a particular sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the invention described herein are, for example, capable of operation in sequences other than those illustrated or otherwise described herein. Furthermore, the terms “comprise,” “include,” “have,” and any variations thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements is not necessarily limited to those elements, but may include other elements not expressly listed or inherent to such process, method, article, or apparatus.
The terms “left,” “right,” “front,” “back,” “top,” “bottom,” “over,” “under,” and the like in the description and in the claims, if any, are used for descriptive purposes and not necessarily for describing permanent relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the invention described herein are, for example, capable of operation in other orientations than those illustrated or otherwise described herein. The term “coupled,” as used herein, is defined as directly or indirectly connected in an electrical, mechanical, or other manner, unless otherwise defined.